nanosuodatus

Nanofiltration (NF), or nanosuodatus in Finnish usage, is a membrane separation process that uses semi-permeable membranes to separate ions and molecules based on size and charge. NF membranes have effective pore sizes around 0.5 to 2 nanometers and typically reject multivalent ions more effectively than monovalent ions, while allowing many small organic molecules to pass. The membranes are usually polymeric, commonly thin-film composite polyamide, and are operated under pressure.

Separation mechanism combines size exclusion with Donnan exclusion (charge effects). The result is selective rejection of divalent and larger ions (such as Ca2+, Mg2+, sulfate) while some small monovalent ions (like Na+) may pass, depending on conditions. Operated as a pressure-driven process, NF requires lower pressures than reverse osmosis, typically 5–40 bar, enabling lower energy consumption. Temperature and ionic strength influence performance.

Typical applications include drinking water treatment (softening and removal of natural organic matter), pretreatment for desalination, wastewater treatment (concentration and removal of color and hardness), and capture of certain pharmaceuticals and pesticides. NF is commonly used as a pretreatment step to RO to reduce scale potential.

Advantages include moderate salt and hardness rejection, relatively lower energy use compared to RO, and good fouling resistance with appropriate pretreatment. Limitations include incomplete removal of monovalent salts, production of concentrating brines, membrane fouling, and higher costs relative to UF. Research directions focus on membrane materials, anti-fouling coatings, and improved selectivity for targeted contaminants.